Pop-up asks about external screen when there is none

A window with “Switch to external screen” pops up (then disappears for a while before popping up again). Several options are shown as complex icons: run only one of the screens, duplicate image on two screens, spread display across both screens as one unit. There is only the default screen on this ASUS VivoBook. Annoying, if not critical. Any suggestion to stop it?

For that external screen I would first check the KDE settings: Settings → System Settings → Display and Monitor. I there one display or multiple?

It would be also good to see the output of “xrandr --listmonitors”.

Only 1 monitor identified by both xrandr (new to me) and KDE settings (I had looked there :-).

Okay, not multiple monitors but you can still very well have multiple virtual desktops.

As this is a laptop it can have touch screen and maybe that is triggering things.
Probably good to check Settings → System Settings → Workspace Behavior although I can not suggest what to look for.

If the window comes really up without interaction, you can use xev to track that down, see monitoring-events-keyboard-mouse-in-x

YES. Used System Settings to configure for only 1 desktop and there were no more notices to switch monitors.